People with lower limb loss or congenital limb absence require a technical substitute that restores biomechanical function and body integrity. In the last decades, mechatronic prostheses emerged and especially actuated ones increased the biomechanical functionality of their users. Yet, various open issues regarding the energy efficiency of powered systems and the impact of user-experience of the prosthesis on technical design remain. As tackeling the latter aspect urgently requires the consideration of user demands, this thesis proposes a novel human-machine-centered design (HMCD) approach for lower limb prosthetics. Further, it contributes to the design and control of elastic (prosthetic) actuation.

The HMCD approach describes a framework that equally considers technical and human factors. Therefore, seven human factors influencing lower limb prosthetic design are determined, analyzed, and modeled using human survey data: Satisfaction, Feeling of Security, Body Schema Integration, Support, Socket, Mobility, and Outer Appearance. Based on the application of quality function deployment (QFD), those factors can be considered as a HMCD focus in systems engineering. As an exemplary application, a powered prosthetic knee concept is elaborated with the HMCD approach. The comparison of the HMCD focus with a purely technical one, which is determined with a control group, reveals distinct differences in the weighting of requirements. Hence, the proposed method should lead to different prosthetic designs that might improve the subjective user-experience. To support this by integrating users throughout the systems engineering process, two concepts for human-in-the-loop experiments are suggested.

As an enabling technology of powered lower limb prostheses, variable (series) elastic actuation and especially such with variable torsion stiffness (VTS) is investigated. Inverse dynamics simulations with synthetic and human trajectories as well as experiments show that the consideration of the actuator inertia is crucial: Only by including it in advanced models, the whole range of natural dynamics and antiresonance can be exploited to minimize power consumption. A corresponding control strategy adapts the actuator to achieve energy efficiency over a wide range of operational states using these models.

The exemplary design of the powered prosthetic knee with respect to the HMCD prioritization of requirements confirms the fundamental suitability of VTS for integration in prosthetic components. In this, considering actuator inertia enables the determination of an optimal stiffness for serial elastic actuation of the human knee during walking that is not found in previous studies. A first simulation considering the changed dynamics of prosthetic gait indicates the potential to reveal lower design requirements. The designed knee concept combines promising biomechanical functionality and long operating time due to elastic actuation and energy recuperation.

Beyond lower limb prosthetics, the proposed HMCD framework can be used in other applications with distinct human-machine interrelations by adjusting the human and technical factors. Likewise, the insights into variable elastic actuation design and control can be transferred to other systems demanding energy-efficient performance of cyclic tasks.